1. Field of the Invention
The present invention relates to an image reading apparatus for reading an original, etc., and an image forming apparatus equipped with this image reading apparatus.
2. Related Art
In this type of image reading apparatus, the reading operation is performed while an original is transported in a sub-scanning direction on a glass, by allowing a light source unit below the glass to illuminate the original and allowing an imaging element such as a CCD (Charge Coupled Device) to scan the original repeatedly in a main scanning direction. The reading operation is also performed while an original is positioned and set on a glass, by allowing the light source unit, the imaging element, etc. below the glass to move in a sub-scanning direction, allowing a light source unit to illuminate the original, and allowing the imaging element to scan the original repeatedly in a main scanning direction. The light source unit for illuminating the original may be of various types, including a fluorescent light or an LED array.
According to the image taking process by this image reading apparatus, the light for illuminating the original is emitted by the light source or the like and reaches an original surface via the light guiding member or the like. The reflected light (reading light) that is reflected by the original surface is guided along a predetermined optical path to the imaging element such as a CCD. In this process, light which reaches the imaging element from an unintended optical path (so-called stray light) causes adverse influences such as noise or ghost in the taken image of the original.
Attempts to reduce such adverse influences by stray light have been made and known in the conventional art. JP 2013-138388 A (hereinafter called “Patent Literature 1”) prevents stray light by placing a light blocking plate in the vicinity of a light guiding member which is arranged closely to the light source. If the angle of emission by the light guiding member deviates from the regular range, the light blocking plate prevents such light from entering a reflection mirror. JP 2004-157174 A (hereinafter called “Patent Literature 2”) provides an elastic light blocking member between a light source and a contact glass on which an original is set. An original is illuminated with light through a slit-like opening formed in the main scanning direction. Then, reading light reflected by the original is allowed to be incident on a mirror from the slit-like opening and is led to the imaging element.
However, stray light to be blocked by Patent Literature 1 is limited to the one which is directly incident on the mirror through the light guiding plate on the light source side. When light is reflected by the original and incident on the mirror (as reflected light, or reading light), it is impossible to prevent a portion of the reflected light travelling along an unintended optical path (as stray light) from reaching the imaging element. Hence, Patent Literature 1 fails to provide sufficient measures against stray light.
In Patent Literature 2, a frame for holding the light source is in contact with the mirror and covers a part of the reflecting surface of the mirror. This structure decreases an area of the mirror on which the reading light is incident. Hence, there are risks of vignetting of the reading light, or deviation of an optical axis of the mirror due to thermal deformation of the frame under the operational heat. Besides, if the mirror and the frame are separated in Patent Literature 2 in order to prevent such vignetting and deviation of the optical axis, it is difficult to block stray light which passes through a gap between the mirror and the frame. In this case, incident light on a side surface of the mirror, which is not the reflecting surface, may also act as stray light.
For a carriage structure in which the light source unit and the reading unit are integrated, it is particularly difficult to provide sufficient measures against stray light. This is because, in comparison with a structure in which the light source unit and the reading unit are separated from each other, the carriage structure has a smaller distance from the slit through which the reading light from the original enters to the CCD serving as the imaging element. It is also because the carriage structure requires a complicated arrangement for the reflective optical system and the optical path in order to reflect the reading light to the imaging element.